The star said he has acquired an American twang which is barely perceptible as a result of spending so much time there with his family.
He tells tonight's Graham Norton Show that he needed the extra help from the dialect tutor to brush up for his role in Tinker Tailor Soldier Spy which won him an Oscar nomination.
Oldman, 55, said: "I have lived in America for so long, my kids are American and that's what I hear around me every day and occasionally weird sounds that I don't notice slip in. I had to brush up my English."
Asked by Norton if there were any plans to return to the characters in the spy drama, he said: "I have no idea, but it would be nice to play Smiley again."
Another guest on tonight's BBC1 show, Nick Frost, spoke about how he once found himself surrounded by armed police who had mistaken him for a gunman.
The actor - soon to be seen in the film Cuban Fury - said he had been preparing for his role in Channel 4 sitcom Spaced.
"In Spaced I was a weapons expert and had to strip a MP5 Machine gun blindfolded so they gave me one to take home - it didn't fire or anything.
"I'd done it a few times when I pulled off the blindfold and there were six armed policemen streaming in and screaming at me. A woman had seen the barrel of the gun sticking out of my bag and called the police.
"My voice went very high and I said, 'I'm an actor'. I was told later that they had blocked the road off and had a sniper and if I had pointed the gun at them, they would have shot me," Frost added.
:: The Graham Norton Show is on at 10.35pm tonight.